The great variety of aliphatic esters and acids which find current use in the industry of flavours and perfumes includes certain .gamma.,.delta.-unsaturated derivatives, some of which are natural occurring. Acids such as pent-4-enoic, hept-4-enoic and 3-methyl-dec-4-enoic have found a particularly advantageous utilization as flavouring ingredients. Among the ester derivatives, special mention should be made for ethyl hept-4-enoate and ethyl oct-4-enoate in their trans isomeric configuration, which compounds have been found to occur in a fraction extracted from passion fruit.
Very numerous are the synthesis known sofar for the preparation of the said compounds and recent investigations have shown the applicability of the Claisen rearrangement to the synthesis of .gamma.,.delta.-unsaturated acids and esters. For instance, Johnson [see: W. S. Johnson et al., J.Am.Chem.Soc., 92, 741 (1970)] has described the preparation of unsaturated esters starting from an allylic alcohol and an alkyl ortho-acetate according to the following reaction scheme: ##STR8## Alk=alkyl R.sup.1, R.sup.2 =H, alkyl
Another process developed by Ireland [See: R. E. Ireland et al., J.Am.Chem.Soc., 94, 5897 (1972), and Tetrahedron Letters 1975, 3975] has recourse to the principle of the rearrangement of an allylic ester enolate, which enolate is formed in situ starting from an allylic ester in accordance with the following reaction scheme. ##STR9## R.sup.1, R.sup.2, R.sup.3 =alkyl
The present invention provides a novel and original solution to the problem of preparing .gamma.,.delta.-unsaturated acids and esters.